Master mixed up class of similar ability (6)
I believe the answer is:
stream
'class of similar ability' is the definition.
'stream' can be an answer for 'class' (stream can mean a class of similarly able pupils). I'm not certain of the 'of similar ability' bit.
'master mixed up' is the wordplay.
'mixed up' indicates anagramming the letters.
'master' with letters rearranged gives 'STREAM'.
Can you help me to learn more?
(Other definitions for stream that I've seen before include "pupils pour out" , "divide pupils into ability groups" , "'Brook, runnel (6)'" , "One small river" , "Brook or rivulet" .)